As intermittent renewable energy sources such as wind and solar increase their share of overall energy production, a method is required to compensate for their intermittency and to match the demand of a power grid in real time. Numerous methods have been discussed to stabilize intermittent renewables, including grid extension to average over larger sets of intermittent assets, demand-side management, ramping of conventional assets, and energy storage, including technologies such as electrochemical storage, such as Li-ion, Na/S, and Na/NiCl2, thermal storage, power to gas, and other energy storage technologies. Flow batteries are one particularly promising technology used to store electrical energy and stabilize power flow from intermittent renewable energy sources. While the most prominent flow battery couple uses vanadium at different oxidation states at each electrode, there are many other couples under consideration, with reactants in the gas, liquid, and solid forms.
One particular flow battery reacts H2 and Br2 to form HBr on discharge. One advantage of this couple is that the H2 reaction is kinetically rapid when catalyzed and the Br2 reaction is kinetically rapid, whether or not the reaction is catalyzed. Rapid kinetics and the ability to obtain components from the related system reacting H2 and O2 in a proton-exchange membrane fuel cell enable the H2/Br2 chemistry to achieve a very high power density. This high power density reduces the area required for a given amount of power and, since the system cost has a significant dependence on the total area over which the reactions are carried out, holds promise for energy storage with reduced costs.
FIG. 1 illustrates a schematic diagram of a conventional H2/Br2 flow battery cell 100 including a number of cell layers included in the cell 100. FIG. 1 illustrates the reactions occurring during battery discharge, though reversing the illustrated reactions results in charging the battery cell 100. Hydrogen gas (H2) is sent through a hydrogen gas channel 104 into a negative electrode 108, at which a porous medium 112 and a catalyst layer 116 are present. The catalyst layer 116 is typically made of Platinum (Pt) to catalyze H2 oxidation on discharge and hydrogen ion (H+) reduction on charge. During discharge, H+ is produced from the H2 gas via the reaction H2→2H++2e−. The H+ passes through a membrane 120 to a positive electrode 124, where it is combined with Br− to form hydrobromic acid (HBr) via the reaction Br2+2H++2e−→2HBr. The membrane 120, in one or more embodiments, is an ion-exchange membrane, such as the cation-exchange membrane Nafion, or a separator with pores through which the H+ passes. On the positive electrode side a solution composed of Br2 and HBr is delivered and flows through a Br2/HBr channel 128 by the positive electrode 124. A porous electrode 124 is always present, though the catalyst 132 layer is optional on the positive electrode side since the kinetics of the Br2 reaction (Br2+2e−→2Br− on discharge and 2Br−→Br2+2e− on charge) are fast even on uncatalyzed carbon. The presence of HBr, which typically dissociates to form H+ and Br—, allows for the conduction of ionic current within the porous electrode. The electrons are passed through an external circuit, where useful work may be extracted (discharge) or added (charge) to the circuit.
FIG. 2 illustrates a prior art flow-by cathode design 140 of a flow battery having an inlet 144, an outlet 148, and a cell area of 10 cm2. Active material is transported from the open flow channels 152 down through the porous electrode layer 156. During operation, electrochemical reactions occur within this porous electrode 156. The cell's membrane sits adjacent to the lower face of the porous electrode.
The cathode in FIG. 2 is a flow-by cathode design, in which active material is transported into a cell through open channels that sit adjacent to the porous electrode. On the liquid side of the H2/Br2 system, this active material is either Br2 (during discharge) or HBr (during charge). Polybromides, such as Br3 and Br5− also serve as reactants during discharge. The active material is then further transported from the open channels into the porous electrode.
While the H2/Br2 system has a high power density, numerous challenges remain, including ensuring safe operation of the battery and achieving a low-cost design for a H2/Br2 battery system. Furthermore, the battery system must be designed to limit degradation of the cell components, which is exacerbated by the crossing of the H2, Br2, and HBr through the ion-exchange membrane that is typically used, as well as the strongly acidic nature of the HBr solutions used for the flow batteries. Finally, the flow battery system must be designed to maintain high concentrations of the active material throughout the electrode.
During normal cell operation, relatively high concentrations of the active material are present throughout the electrode. Because concentrations are high, the electrochemical reactions occur without incurring significant voltage penalties. However, when the battery is either fully charged or fully discharged, the concentration of the active materials tends toward relatively small values. In such instances, the cell voltage is reduced dramatically due to the low concentrations of the active materials.
In general, active material can be transported to reactive areas of a flow battery's porous electrode by diffusive, convective, or migration processes. However, when high current densities are needed, diffusive transport cannot be relied upon as a transport mechanism because it is a relatively slow process, and migration is also limited by the intrinsic mobility of species within a solution. Rather, only convective transport can provide sufficient concentrations of active materials to the electrode reaction zones when high current densities are desired.
Flow batteries and flow battery systems are only operated successfully if sufficient concentrations of reactant materials are transported to electrochemical reaction zones on both sides of the battery's cell membranes. During operation of the battery, it is possible that transport of active material through regions of the electrode slows, and the active material that has been electrochemically depleted in these regions may not be replenished with fresh active material from the system storage tanks. The regions in which the electrochemically depleted material is not replenished experience low active material concentrations, and are referred to herein as “dead zones.” Low active material concentrations in some regions can result in reduced performance of the battery, for example causing a decrease in the overall cell voltage during discharge, an increase in the cell voltage during charge, or local changes in the porous electrode's potential field. Changes in the cell voltage negatively impact the efficiency of the battery cell's energy storage capabilities. Furthermore, local changes in the electrode potential can cause cell corrosion and degradation.
Electrical energy is required to operate pumps to convectively transport reactant materials to reaction zones. It is desirable to minimize the power required to transport the materials to the electrochemical reaction zones, since power used by the pump reduces the overall energy efficiency of the flow battery system.
Consequently, what is needed is a flow battery and flow battery system which operates efficiently and delivers sufficient power upon demand, including peak demands of power in various conditions. Additionally, what is needed is a flow battery system reducing the formation of regions having slow transport processes and depleted levels of active material during normal cell operation, while balancing the power drain from operating pumps that increase the transport processes in the flow battery system.